1. Field of the invention
This invention relates to production of three-dimensional objects by photoforming, and more particularly to the controlled application of thin flat layers of a photoformable composition accurately and quickly to a platform or previously photoformed layer(s) to accomplish said production with layers of improved flatness, accuracy and integrity.
2. Description of Related Art
Many systems for production of three-dimensional modeling by photoforming have been proposed European Patent Application No. 250,121 filed by Scitex Corporation Ltd., on Jun. 6, 1987, which discloses a three-dimensional modeling apparatus using a solidifiable liquid, and provides a good summary of documents ertinent to this art. U.S. Pat. No. 4,757,330 (C. W. Hull), issued on Mar. 11, 1986, describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction, successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, being automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process. U.S. Pat. No. 4,752,498 (E. V. Fudim), issued on Jun. 21, 1988, describes an improved method of forming three-dimensional objects, which comprises irradiating an uncured photopolymer by transmitting an effective amount of photopolymer solidifying radiation through a radiation transmitting material which is in contact with the uncured liquid photopolymer. The transmitting material is a material which leaves the irradiated surface capable of further crosslinking so that when a subsequent layer is formed it will adhere thereto. Using this method, multilayer objects can be made.
Publication "Automatic Method for fabricating a three-dimensional plastic model with photohardening polymer" by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773, November 1981, describes a method for automatic fabrication of a three-dimensional plastic model. The solid model is fabricated by exposing liquid photo-forming polymer to ultraviolet rays, and stacking the cross-sectional solidified layers. Publication "Solid Object Generation" by Alan J. Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, August 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, simple three-dimensional objects from information stored in computer memory. A good review of the different methods is also given by a more recent publication, titled "A Review of 3D Solid Object Generation" by A. J. Herbert, Journal of Imaging Technology 15: 186.varies.190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photoformable composition with a laser beam according to a desired pattern and building a three-dimensional model layer by layer. In addition to various exposure techniques, several methods of creating thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
Current methods of coating suggested thus far, however, have drawbacks in that they are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, or they do not effectively prevent damage or distortion to previously formed layers during the successive coating process and they involve coating only liquid formulations of preferably low viscosity. Furthermore, they omit to recognize very important parameters involved in the coating process such as the effects of having both solid and liquid regions present during the formation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photoformed layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as hydrogen bonds and substantially stronger forces such as mechanical bonds and vacuum or pressure differential forces on those thin layers and on the part being formed.
The Hull patent, for example describes a dipping process where a platform is lowered either one layer thickness or is dipped below the distance of one layer in a vat then brought up to within one layer thickness of the surface of the photoformable liquid. Hull further suggests that low viscosity liquids are preferable, but for other practical reasons, the photoformable liquids are generally high viscosity liquids. Although theoretically most liquids will flatten out eventually, high viscosity liquids and even low viscosity liquids take an inordinate amount of time to flatten to an acceptable degree especially if large flat areas are being imaged and if the liquid layer thickness is very thin. Regions where previous layers consist of solid walls surrounding liquid pools further compounds the flattening process of the thin liquid layer coating. In addition, motion of the platform and parts, which have cantilevered or beam (regions unsupported in the Z direction by previous layer sections), within the liquid creates deflections in the layers, contributing to a lack of tolerance in the finished part.
The Munz patent (U.S. Pat. No. 2,775,758, issued in 1956) and Scitex application describe methods by which the photoformable liquid is introduced into the vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have all the problems of the Hull methods except that the deflections of the layers during coating is reduced.
The Fudim patent describes the use of a transmitting material, usually rigid and coated or inherently unlikely to adhere to the solidified photopolymer, to fix the surface of the photopolymer liquid to a desired shape, assumably flat, through which photopolymers of desired thickness are solidified. The methods described by Fudim do not address the problems inherent in separating such a transmitting material from a photopolymer formed in intimate contact with the surface of the transmitting material. Whereas the effects of chemical bonding may be reduced significantly by suitable coatings or inherently suitable films, the mechanical bonds along with hydrogen bonds, vacuum forces, and the like are still present and in some cases substantial enough to cause damage or distortion to the photopolymer during removal from the transmitting material surface. Furthermore, evaluations made by the Applicants indicate that the forces, resisting the separation or even sliding off the solidified layer having been exposed in intimate contact with the suitably non-adhesive transmitting material, are capable of damaging the solidified layer, especially when surrounded by photoformable liquid and even more especially when the solidified layers are thin. No method is described in the Fudim patent to eliminate these problems.
The instant invention removes these vexing problems by utilizing a stretched elastomeric film in contact with the photoformable composition, imaging through said elastomeric film, changing adequately the tension on the film to break the weak bonds between the solidified layer and the film, forming a new layer, and repeating the steps until the desired three-dimensional object is ready. Thus, the present invention provides a method and apparatus for quickly producing layers of a deformable and photoformable composition, which are flat, and with which previously exposed and solidified layers have improved flatness, accuracy and integrity during the process. Preferably, the solidified layers according to this invention have a thickness of less than 0.030".
Deformable compositions are those, which under pressure alone, or under both pressure and temperature take the form of the mold they are in, or the shape of the surfaces they are pressed by. The layer is considered to be solidified when hardened, or polymerized, or crosslinked by the radiation, or even more generally when its viscosity is increased by the radiation, so that higher pressure and/or temperature will be needed for it to be re-deformed, if at all possible. Liquids are the preferred deformable compositions, as they deform by the pressure of their own weight, thus being free flowing.